In a modern information communication network, mathematical encryption for disrupting transmission information by a mathematical method is used in order to prevent a third party from eavesdropping on a communication message. Recently, development has begun on physical encryption aimed to achieve a higher degree of security by using the physical phenomena of a signal system in means of communication.
Among physical encryption, an encryption for using the quantum noise inevitably generated when an optical signal is received to achieve a high degree of security in terms of encryption is called Yuen encryption or quantum stream cipher, which is based on the Yuen-2000 protocol (called Y-00). With this encryption, a set of two signals for transmitting an information bit is called a base, and a plurality (M) of bases are prepared; a pseudorandom bit sequence where an initial key has been elongated by a pseudorandom bit generator is used to irregularly select the bases, and an optical signal corresponding to a selected base is used to transmit the information bit (called “plain text” in cryptology). The receiver uses the same secret key and pseudorandom bit generator as a transmitter, which are synchronized with the transmission side, to identify the binary signals of 1 and 0.
In Yuen encryption, an eavesdropper who is unaware of the key will not know which base is being used, and therefore it is necessary to identify 2M signals. In such a case, the error properties of the identification of the two values of a legitimate receiver will be substantially zero, and when a signal and noise effects are designed such that the error properties of the 2M identifications of the eavesdropper become severely deteriorated, an advanced concealment effect against the eavesdropper is obtained. Obtaining the concealment effect in this manner is called a principle of generating security gain based on a communication format and the noise effect.
Known communication formats for implementing Yuen encryption include the optical phase modulation format disclosed in Non Patent Literature 1 and the optical intensity modulation format disclosed in Non Patent Literature 2. In these formats, an optical signal corresponding to a base is deployed in conformity with a single relational formula. With the optical phase modulation format, deployment is made on the phase plane at positions where the circumference is spaced by the amplitude A equally apart with 2M signals. With the optical intensity modulation format, optical signals are deployed at 2M regular intervals using an intermediate point between a maximum intensity and a minimum intensity as a reference, or reducing the interval in a linear manner from a maximum to a minimum. Depending on applications, various other deployments have also been proposed.
The above-described signal deployment is an innovation for causing the effects of quantum noise to be realized evenly. Security (Q) relative to a secret key against an eavesdropper is readily assessed by the following formula (Non Patent Literature 2).
[Math. 1]Q=ΓK/logM  (1)
In the formula, Γ is the number of signals hidden by quantum noise, K is the length of the secret key, and M is the number of bases. The possibility of a secret key when Γ=M is not reduced at all even though the number of observations is increased, and therefore, this is called information-theoretic security. With the phase modulation format, the quantum noise becomes equivalent to vacuum noise, and is therefore very low, making it difficult to increase Γ, but quantum noise in the intensity modulation format has a characteristic of appearing as quantum shot noise, and therefore there is a large amount of quantum noise and it is readily possible to increase Γ.
FIG. 1 is a diagram illustrating the configuration of Yuen encryption using optical intensity modulation according to the conventional art as recited in Non Patent Literature 2. The following provides a description of the fundamental principle of the Yuen encryption device according to the optical intensity modulation format, with reference to FIG. 1.
In FIG. 1, the conventional encrypted communications device is a configuration in which an optical transmission device 10 and an optical reception device 20 are connected by an optical communication pathway 30 such as an optical fiber. The optical transmission device 10 is provided with a carrier wave generation unit 11, an M-ary intensity modulation unit 12, a pseudorandom bit generation unit 13, a base selection control unit 14, and a transmission data generation unit 15. The optical reception device 20 is provided with a photodiode 21, an intensity determination unit 22, a signal determination unit 23, a pseudorandom bit generation unit 24, and a base selection control unit 25. The pseudorandom bit generation unit 13 of the optical transmission device 10 and the pseudorandom bit generation unit 24 of the optical reception device 20 have substantially the same configuration and function. The base selection control unit 14 of the optical transmission device 10 and the base selection control unit 25 of the optical reception device 20 have substantially the same configuration and function.
The carrier wave generation unit 11 is composed, for example, of a laser diode, and outputs a predetermined optical carrier wave. The transmission data generation unit 15 generates transmission data configured by information of “1”s and “0”s based on information to be transmitted. The pseudorandom bit generation unit 13 generates a binary pseudorandom bit sequence, specifically, a binary Running key sequence, based on an initial key K. The base selection control unit 14 divides the binary Running key sequence into blocks at every log M bit, thus converting to a decimal Running key corresponding to each of the blocks. The base selection control unit 14 selects one base from a base group in conformity with the Running key, and indicates the same to the M-ary intensity modulation unit 12 as base information. The M-ary intensity modulation unit 12 modulates the intensity of the optical carrier wave with transmission data using an optical intensity corresponding to the base indicated by the base information, and outputs the same to the optical reception device 20 via the optical communication pathway 30.
The photodiode 21 receives an intensity modulation optical signal outputted from the optical transmission device 10 via the optical communication pathway 30. The pseudorandom bit generation unit 24 generates a binary Running key sequence based on the initial key K. The base selection control unit 25 divides the binary Running key sequence into blocks at every log M bit, thus converting to a decimal Running key corresponding to each of the blocks. The base selection control unit 25 selects one base from the base group in conformity with the Running key, and indicates the same to the signal determination unit 23 as base information. The signal determination unit 23 controls how a received signal is determined, based on the base information indicated by the base selection control unit 25, extracts information of “1”s and “0”s contained in the signal, and outputs the same as reception data. Specifically, provided is a function for determining the same to be 1 or to be 0 when the received signal reaches higher or lower than a threshold value.
In the aforementioned conventional Yuen encrypted communication device, the deployment of the base group, specifically, the optical signals corresponding to each of the bases, used in the base selection control units 14 and 25 is an important element for determining the level of encryption.